Background: Onychomycosis is sufficiently prevalent to be seen and treated by primary care physicians. The diagnosis of onychomycosis is most often confirmed from nail specimens by microscopy and fungal culture done at a central laboratory; these are relatively expensive tests with a turnaround time of a month or more. This study was conducted (1) to evaluate the use of in-office dermatophyte test medium (DTM) culture, and (2) to determine the epidemiology of onychomycosis in a large, nationwide sample of patients who were not participants in a clinical trial. Methods: A nationwide sample of primary care physicians and podiatrists enrolled 670 patients with clinical signs of toenail onychomycosis. Dermatophyte test medium cultures were performed in the office and the results were compared with fungal cultures performed by a central laboratory. Results: Central laboratory fungal cultures were positive in 44% (n=297) of patients and DTM cultures in 51% (n=345). Dermatophytes accounted for 93% of the confirmed infections and nondermatophyte molds the rest. In the 617 patients with paired dermatophyte test medium and laboratory fungal culture results, the 2 tests were in agreement (both positive or both negative) in 68% of cases (κ, 0.37; asymptotic SE, 0.04; 95% confidence interval, 0.299-0.441). Conclusions: A DTM culture is a relatively rapid, easy, and inexpensive method to confirm dermatophyte infections in patients with signs of onychomycosis in the primary care setting. Because the available drugs for treating onychomycosis are effective against all dermatophyte species, the confirmation of dermatophyte infection, without further identification of genus and species, is sufficient evidence to begin treatment.